Attitude Constrained Robust Explicit Guidance for Terminal Phase of Autonomous Lunar Soft-Landing

An attitude-constrained jerk-minimizing optimal explicit terminal guidance is presented in this paper for the terminal phase of a multi-phase lunar soft-landing. An analytic expression for the acceleration vector serves as the guidance command that can be computed by low-speed onboard processors in real-time. High terminal accuracy in each phase is achieved by enforcing ‘hard terminal constraints’ on the position, velocity, and acceleration (hence attitude) of the spacecraft. The initial conditions on position, velocity and acceleration are also enforced as hard constraints to ensure a smooth handover between consecutive phases. The required time-to-go is computed to minimize the deviation of the acceleration from the average of initial and final values. This results in a very smooth variation of the acceleration vector (hence attitude as well, owing to the strap-down nature of the thruster), starting from the applicable initial value to the desired final value in the segment. Along the trajectory, compatibility of the recomputed time-to-go in the absence of additional perturbation is maintained by auto-adjusting a tuning parameter. Moreover, if necessary, the time-to-go is adjusted once again to prevent altitude excursion beyond a pre-selected safety margin. This process of careful online auto-adjustment of the time-to-go, followed by using it to recompute the guidance command, makes the guidance operate in closed-loop with significant robustness. Consequently, it enables a larger capture region and introduces the capability to recover from partial failures, which are critical requirements for such challenging missions.